In general, a grain-oriented silicon steel sheet is used as an iron core of electrical apparatuses such as transformers. The grain-oriented silicon steel sheet must have high magnetic flux density (represented by a value B.sub.8) and low iron loss (represented by W.sub.17/50) as magnetic properties.
In order to improve magnetic properties of the grain-oriented silicon steel, first, the &lt;001&gt; axis of secondary-recrystallized grains in the steel sheet must be highly oriented in the rolling direction. Secondly, impurities and precipitates that remain in the end product must be minimized.
Since the basic production technique of the grain-oriented silicon steel sheet by two-stepped cold rolling method was suggested by N. P. Goss, various improvements have been attempted. As a result, magnetic flux density and iron loss have been enhanced year by year.
Typical improvement techniques include a method disclosed in Japanese Patent Publication No. 51-13469 in which Sb, and MnSe or MnS are used as inhibitors, and methods disclosed in Japanese Patent Publication Nos. 33-4710, 40-15644, and 46-23820 in which AlN and MnS are used as inhibitors. By these methods, products with a high magnetic flux density B.sub.8 of more than 1.88 T have become obtainable.
In order to obtain products with higher magnetic flux density, other methods have been disclosed, including, for example, Japanese Patent Publication No. 57-14737, in which Mo is added to a raw material, and Japanese Patent Publication No. 62-42968, in which, after Mo is added to a raw material, quenching is performed after intermediate annealing immediately before final cold rolling. By these methods, a high magnetic flux density B.sub.8 of 1.90 T or more and a low iron loss W.sub.17/50 of 1.05 W/kg or less (product sheet thickness: 0.30 mm) have been obtained. However, there is room for improvement with respect to further enhancement of low iron loss.
In particular, demands for absolute decrease in power loss have risen significantly since the recent energy crisis. Therefore, further improvement in iron core materials also has been desired, and products with a sheet thickness of 0.23 mm or less are now widely used.
In addition to the metallurgical methods described above, as disclosed in Japanese Patent Publication No. 57-2252, a method for reducing iron loss by artificially decreasing 180.degree. magnetic domain width (magnetic domain refining technique) has been developed, in which the surface of a finishing-annealed steel sheet is irradiated with laser or is irradiated with plasma (B. Fukuda, K. Sato, T. Sugiyama, A. Honda, and Y. Ito: Proc. of ASM Con. of Hard and Soft Magnetic Materials, 8710-008, (USA), (1987)). By this technique, the iron loss in the grain-oriented silicon steel sheet has been greatly reduced.
Annealing at high temperatures, however, ruins the iron loss improvement effect caused by the magnetic domain refining technique using laser irradiation or the like. Accordingly, the usage of the product manufactured by this technique is limited to laminated iron-core transformers which generally do not require stress-relief annealing.
Therefore, as a magnetic domain refining technique having a sufficient iron loss improvement effect to withstand stress-relief annealing, a method has been industrialized, in which linear grooves are formed on the surface of a finishing-annealed grain-oriented silicon steel sheet and domain refining is performed using the demagnetizing field effect by the grooves (H. Kobayashi, E. Sasaki, M. Iwasaki, and N. Takahashi: Proc. SMM-8., (1987), P.402).
Apart from this, a method has been developed and industrialized (as disclosed in Japanese Patent Publication No. 8-6140), in which grooves are formed by localized electrolytic etching onto the final cold-rolled grain-oriented silicon steel sheet to refine magnetic domains.
Besides the grain-oriented silicon steel sheet, amorphous alloys, which are disclosed in Japanese Patent Publication No. 55-19976 and in Japanese Patent Laid-Open Nos. 56-127749 and 2-3213, have been noted as materials for general power transformers, high-frequency transformers, and the like.
Such amorphous materials have excellent iron loss in comparison with general grain-oriented silicon steel sheets. However, there are many disadvantages in practical use, such as, 1) lack of thermal stability, 2) poor lamination factor, 3) difficulty in cutting, and 4) high cost of fabrication of the transformers because of excessive thinness and brittleness. Accordingly, the amorphous materials have not been used in large quantity.
On the other hand, the present inventor has disclosed that ultra-low iron loss can be obtained by forming a tensile coating of at least one of either a nitride or a carbide of Si, Mn, Cr, Ni, Mo, W, V, Ti, Nb, Ta, Hf, Al, Cu, Zr, and B onto the grain-oriented silicon steel sheet, which has been smoothed by polishing, by means of dry plating, for example, CVD, ion plating, ion implanting, and sputtering, as disclosed in Japanese Patent Publication No. 63-54767 and so on. By the production method described above, grain-oriented silicon steel sheets having excellent iron loss are obtainable as materials for power transformers, high-frequency transformers, and the like. However, this does not sufficiently meet the recent demand for the enhancement of low iron loss.
The present invention advantageously satisfies the recent demand for the enhancement of low iron loss, and it is an object of the present invention to provide a grain-oriented silicon steel sheet which enables further reduction in iron loss in comparison with the conventional art.